The present invention relates to a semiconductor device and a manufacturing technique for the same. Particularly, the present invention is concerned with a semiconductor device wherein a bonding pad of a semiconductor chip and a lead frame are coupled together using a metallic ribbon, as well as a technique effectively applicable to the manufacture thereof.
A semiconductor chip having formed thereon power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) each used for example as a power control switch or a charge/discharge protecting circuit switch in a portable information device, is sealed in a small-sized surface mount package such as a flat lead package or a SOP-8.
A back surface of the above semiconductor chip configures a drain of the power MOSFETs and is bonded onto a die pad portion of a lead frame through a conductive adhesive such as Ag paste. In a top layer of a main surface of the semiconductor chip there is formed a source pad coupled to sources of the power MOSFETs and a gate pad coupled to gate electrodes of the same transistor. The source pad is formed at a wider area than the gate pad in order to decrease ON resistance of the power MOSFETs.
Plural leads which configure external coupling terminals are exposed to the exterior of molding resin which seals the semiconductor chip. These leads comprise source, drain and gate leads. The drain leads are integral with the die pad portion and are electrically coupled to the back surface of the semiconductor chip (drain of the power MOSFETs) mounted on the die pad portion.
Recently, in the surface mount package of the above structure, the technique of coupling a source pad and source leads with use of a flexible metallic ribbon has been adopted practically in order to decrease ON resistance of the power MOSFETs.
The metallic ribbon is formed for example by Al (aluminum) foil or Cu (copper) foil having a thickness of several hundred μm or so and the width thereof is generally 1 mm or so though it differs depending on the source pad width. For coupling the metallic ribbon to the source pad or source leads there is used a wedge bonding method which utilizes ultrasonic vibration.
The metallic ribbon is advantageous in that the ribbon width is much larger than the diameter of Au (gold) wire and that therefore even with a single metallic ribbon it is possible to ensure a sufficient coupling area for the source pad and ON resistance of the power MOSFETs can be greatly reduced as compared with the case where both source pad and source leads are coupled together using plural Au wires. Besides, there also is an effect that the cost of the package material can be reduced because the ribbon is formed by Al which is cheaper than Au.
Patent Document 1 discloses an improved technique of a wedge tool used for coupling the above metallic ribbon. In a lower surface of the wedge tool described in Patent Document 1 there are formed plural grooves or notches in parallel with the extending direction of the metallic ribbon. Therefore, when the wedge tool is brought into pressure contact with the metallic ribbon disposed on a semiconductor chip, only a part of the tool lower surface comes into contact with the metallic ribbon. Consequently, the propagation of excessive ultrasonic vibration energy from the wedge tool to the surface of the semiconductor chip can be prevented and hence the occurrence of damage such as a crack or a fissure in the semiconductor chip is decreased.
In Patent Document 2 is disclosed a wedge tool for ribbon bonding wherein plural projections and plural grooves each formed between adjacent such projections are formed on a pressure bonding surface.
In Patent Document 3 is disclosed a wedge tool for ribbon bonding wherein plural projections are formed on a pressure bonding surface. In each of the plural projections, two side faces opposed to each other are inclined relative to the pressure bonding surface, while other two side faces adjacent to the two side faces are nearly perpendicular to the pressure bonding surface.
In Patent Document 4 is disclosed a technique wherein projections are formed by knurling on a pressure bonding surface of a wedge tool for ribbon bonding, then the projections are pushed onto a metallic ribbon in ultrasonic bonding to apply a pushing load and ultrasonic vibration to the metallic ribbon, thereby forming ultrasonically bonded portions corresponding to a required energizing capacity dispersedly in an overlapped surface area between a heat spreader and the metallic ribbon.
In Patent Document 5 is disclosed a technique wherein one or plural loops are formed in a metallic ribbon when bonding the ribbon to a pad.